r77477 has broken compilation of Boost.Context on trunk for me when using MSVC and targeting x64 (it spits out a "platform not supported" error). I believe this is because MSVC does not define any of the preprocessor directives that are being checked now (instead of _WIN64). I believe the fix for this is to check _M_AMD64 and/or _M_X64. http://msdn.microsoft.com/en-us/library/b0084kay(v=vs.110).aspx _M_X64: Defined for x64 processors. _M_AMD64: Defined for x64 processors. This patch fixes the issue for me: http://dl.dropbox.com/u/74751/cpp/boost/patches/context-msvc-x64-fix-2012032... Thanks.